Technical Field
The present invention generally relates to Radio Frequency (RF) front end circuits for receiver-transmitter (Rx-Tx) devices coupled to a single antenna. Such RF front end circuits enable to switch the Rx-Tx devices from a reception mode to a transmission mode.
Related Art
New developments in RF technology ask for better integration of several blocks into a single System-On-Chip (SOC). A RF front end circuit includes at least one power amplifier (PA) for transmitting a signal to the antenna, at least one low noise amplifier (LNA) for receiving a signal through the antenna and switches arranged to cause the circuit switch from a reception mode to a transmission mode or vice-versa. Existing technologies allow integration of one LNA and several PAs together using RF switches. This allows multiple power amplifiers being coupled to one common antenna. As only one PA supplies power to the antenna at the same time, RF switches isolate the other PA or PAs by disconnecting them from the antenna. FIG. 1 diagrammatically shows a classical approach of such a RF front end circuit including multiple PAs and at least one LNA, and comprising RF switches SW1 to SW3 to connect the PAs and the LNA to a common impedance matching network MN and to an antenna Ant.
However RF switches generally use GaAs or SOI technologies which are expensive. This design is not suitable for applications in low cost CMOS technology.
In CMOS technology, some circuits have been proposed to couple a PA and a LNA to a common antenna. Such circuits are generally based on DC-switches and passive components which are easily designed in CMOS technology.
In existing solutions using CMOS technology, one PA and one LNA operating according to a common communication standard share a connexion to one and the same antenna. In such circuits, a common impedance matching network may be arranged between the two blocks and the antenna. The common impedance matching network is optimized for the LNA and an additional impedance matching network is added for the PA. The PA and the LNA may be put into an ON state or an OFF state through use of DC-switches. At any time, one of the PA or LNA is put in an OFF state so that it does not interact with the antenna. Accordingly, the other element, LNA or PA respectively, may interact with the antenna absent any interference from the other element which is in the OFF state.
The additional impedance matching network may comprise a low pass filter or a high pass filter coupled in series between the output of the common impedance matching network and the input of the PA.
A challenging evolution leads to share a common pin between several PAs operating at different output power and one LNA. Thus, each PA needs a particular impedance matching network different than those needed by the other PAs, to efficiently transmit a signal.
As an example, one may want to design RF front end circuits in which the antenna is coupled to a LNA for receiving data and both to a first PA for Wi-Fi communication with an output power of ca. 21 dBm, and to a second PA for Bluetooth communication with a lower output power of ca. 13 dBm.
The issue with coupling different PAs with different output power to a common antenna is that the first PA which outputs the maximum power imposes the design of the impedance matching network and thus the common load. However the common load is not high enough for enabling the second PA to output a smaller power without degrading its efficiency.
Thus, a second additional impedance matching network has to be added specifically to increase the load presented to the input of the second PA. However, as the second PA is configured to output less power, such impedance matching network provides a second impedance to the second PA which is higher than the one needed by the first PA. Even if present on a branch of the circuit other than the branch including the first PA, components of the second additional impedance matching network may influence the performance of the first PA, due for instance, to some parallel coupling.
Therefore, in circuits integrating multiple PAs using close-by operating frequencies, it is desirable to isolate the second additional impedance matching network of a second PA from the first PA which outputs the higher power.
There is therefore a need to provide improved RF front end circuits which may connect multiples PA to a common antenna without using RF switches.